The ribosomal and nonribosomal proteins of HeLa cell nucleolar ribosomal precursor particles are being studied in several ways, including the order of addition of the proteins to the RNA and variations in the amount and stability of each protein in response to changes in the growth rate of the cells. An attempt to identify the human chromosomes that code for several of these proteins, using high-resolution polyacrylamide gel electrophoresis to distinguish human from hamster nucleolar proteins in hybrid cells, has been initiated. The functions of HeLa ribosomal proteins S3a and Sa, either or both of which may be involved in the binding of messenger RNA to ribosomes, are being investigated.